negative index for the sequence

[Hiroshi G. Okuno <Okuno@SUMEX-AIM.ARPA>]

 > Well, SUBSEQ must check the length if it is defined to SIGNAL an error for
 > an out-of-bound index.
 >
 > --Guy

If the length check is performed in SUBSEQ, a negative index may be
considered as a convention of

	-n == (- (length <object>) n)   where n > 0.

For example,

(subseq #(a b c d e) -3 -1) = (subseq #(a b c d e) 2 4) = #(c d e)

Of course, if the non-negative index which corresponds to a negative one
exceeds the bound of the sequence, an error is signalled.

This convention is very useful for string manipulations.  In fact,
a negative index is supported in TAO and used extensively in a
kana-kanji conversion programs.

- Gitchang -
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